Not sure if people already posted this but if you faction change the starting zone quest The Legion Returns gets reset. You still keep your progress on Broken Isles and have your Dalaran hearthstone. Everything is the same.
The quest gives you Orgrimmar Portal Stone for Horde and Stormwind Portal Stone for alliance. They are a 10 second hearth CD and can be used anywhere.
You could probably even get a faction refund and probably have the quest without paying anything.
again sorry if this was already posted, I just thought it was pretty cool; downside for horde is your zoned out of outside orgrimmar, but if you dont have an ego I don't think this will affect you